Laser Induced Phonons (LIPS) are produced by time-coincident laser pulses intersecting inside a sample, setting up an optical interference pattern, i.e., alternating intensity peaks and nulls. Energy deposited into the system via optical absorption or stimulated Brillouin scattering results in the launching of counterpropagating ultrasonic waves (phonons) whose wavelength and orientation match the interference pattern geometry. The mechanism by which LIPS ultrasonic waves are generated depends upon whether the sample is optically absorbing or transparent at the excitation wavelength. If the excitation pulses are absorbed e.g. into high-lying vibronic levels, rapid radiationless relaxation and local heating at the interference maxima (the transient grating peaks) occurs. Thermal expansion then drives material in phase away from the grating peaks and toward the grating nulls, setting up counterpropagating waves.
In samples which are transparent at the excitation wavelength, optical energy is coupled directly into the sample's acoustic field via stimulated Brillouin scattering. This process takes advantage of the inherent spectral line width in 100-picosecond (psec) excitation pulses. Higher-frequency photons from each pulse are annihilated to create lower-frequency photons in the opposite pulse and phonons of the difference frequency and wave vector in the medium. Counterpropagating waves (a standing wave) are thus produced.
In either case the acoustic wave propagation, which continues after the excitation pulses leave the sample, causes time-dependent, spatially periodic variations in the material density, and since the sample's optical properties (real and imaginary parts of the index of refraction) are density-dependent, the irradiated region of the sample acts as a Bragg diffraction grating. This propagation of the optically excited ultrasonic waves can be optically monitored by time-dependent Bragg diffraction of a variably delayed probe laser pulse.